Mobile phone tracking refers to attaining of the current position of a mobile phone (user equipment (UE)), stationary or moving. Localization may occur either via multilateration of radio signals between several radio towers of the network and the UE, or simply via GPS. Localization-based systems can be broadly divided into network-based, handset-based, SIM-based, and/or hybrid. Network-based positioning techniques utilize the service provider's network infrastructure to identify the location of a UE. The advantage of network-based positioning techniques is that they can be implemented non-intrusively, without affecting the UE.
In network-based positioning, the UE position is estimated based on timing measurements of the sounding reference signal (SRS) taken at different Location Measurement Units (LMUs), along with the knowledge of the geographical coordinates of the LMUs. The time required for an uplink SRS signal transmitted by a UE to reach an LMU is proportional to the length of the transmission path between the UE and the LMU. Therefore, the LMU is able to calculate the UE's distance by uplink SRS timing measurements. Typically, a set of LMUs (e.g., three LMUs) can be used to sample the SRS at the same time to estimate the UE's location.
In 3GPP LTE wireless communication systems, two types of SRS are defined. A first type of Periodic SRS (p-SRS) is used for obtaining long-term channel information. Before a UE starts p-SRS transmission, its serving base station (eNodeB) needs to allocate periodic SRS resources and configure SRS parameters to the UE by higher layer signaling. A second type of Aperiodic SRS (ap-SRS) is triggered by uplink grant via physical downlink control channel (PDCCH). Once triggered, the UE transmits a sounding sequence in a pre-defined location. Typically, in addition to obtaining channel information, p-SRS is also used for network-based positioning.
In order to obtain uplink SRS measurements, the LMUs need to know the characteristics of the uplink SRS signal transmitted by the UE for the period required to calculate uplink measurements. These characteristics have to be static over the periodic transmission of SRS. Hence, the eNodeB will be requested communicate to an Enhanced Serving Mobile Location Center (E-SMLC) these SRS characteristics so that the E-SMLC can configure the LMUs accordingly. The eNodeB is responsible for allocating the target UE SRS resources and communicating the SRS configurations to the E-SMLC. If the eNodeB determines that it will be unable to configure the target UE, then the eNodeB sends a failure indication to the E-SMLC.
The 3GPP LTE specification has defined various SRS parameters (e.g., cell-specific SRS parameters and UE-specific SRS parameters) for uplink SRS transmission for each target UE. For example, different SRS parameters are defined for SRS sequence-group hopping and sequence hopping, SRS resource allocation, SRS frequency hopping, UE antenna selection, and TDD-specific parameters, etc. To achieve accurate SRS measurements, the serving eNodeB needs to determine what part of the SRS configuration data of the target UE needs to be transferred from the eNodeB to the E-SMLC and from the E-SMLC to the LMUs.
Furthermore, according to 3GPP LTE specification, a UE shall abandon the transmission of SRS in a subframe if the coincidence of some specific uplink channels in the same subframe occurs. This SRS dropping is unknown to the LMUs because the rules governing the SRS dropping are quite complex and it is unlikely for the eNodeB to transfer all the information related to the SRS dropping rules to the LMUs. In this case, the LMUs may try to measure the SRS from the target UE even if the SRS is not transmitted at all. Therefore, handling SRS dropping is required to avoid causing performance degradation of the network-based positioning technique.